Production systems are generally installed for relatively great lengths of time, 20 years for example. While they are installed and during production operations, they undergo outside stresses such as wave motion, current, wind. . . .
The floating support is usually anchored statically to the sea bottom by means of a series of chains or of vertical or oblique taut lines. In both cases, it retains a certain freedom of motion on and along various axes, that range from some centimetres to some metres for vertical displacements due to the wave motion, known in this field as heave, and up to several ten metres in the horizontal plane, known as surge, sway and slow drift. Rotations around the horizontal axes, roll/pitch, and around the vertical axis, yaw, depend on the dimensions of the floating support, on its anchor means, and on the wave motion, current and wind conditions.
Conventionally, in such installations, the riser pipes are fastened on the one hand to a subsea structure placed on the bottom and generally including several wellheads, and on the other hand they are directly or indirectly connected to a floating structure by means of suitable devices. These connecting devices make the riser pipes more or less dependent on the floating support and therefore on its displacements.
Using flexible risers is particularly well-suited for this type of displacements. They respond very well to the motions at the head (in the vicinity of the connection with the floating support) and the bottom contact is well controlled. The numerous applications of flexible risers implemented throughout the world and in the offshore sphere show that the fatigue aspects for this type of riser can be considered to be sufficiently controlled.
Dimensioning of flexible risers must take account of traction and collapse among other criteria. In deep seas, knowing that a flexible riser is generally heavier than a rigid one, combination of the two aforementioned criteria can become difficult to control.
For entirely rigid and practically vertical risers, suspension systems better known as tensioning systems are generally used so that these displacements can be borne by the riser pipe. Hydraulic tensioning or passive float tensioning systems that keep the riser pipe under more or less constant tension and independent of the motions of the support are for example used. These systems can become very cumbersome for risers at great depths.
Rigid riser systems known as catenary, that can be used in deep seas, use the flexibility of the metal over a great riser length in order to give them a shape similar to the conventional shape of the flexible. These risers can possibly be without tensioning means, but they have two major drawbacks:                a great horizontal distance is required between the riser head and the subsea wellhead,        the fatigue at the separation point is critical.        
The prior art also describes various layouts notably intended to take up the motions of the floating supports by combining rigid part and flexible part for the riser system.
For example, hybrid risers such as those used in U. S. Pat. No. 4,661,016 or the Mobil/IFP Compliant riser presented for example in “Applications of Subsea Systems” (Goodfellow Associates Ltd, 1990) consist of a riser or of a tower of rigid risers extending from the sea bottom to a certain given depth. This depth is preferably below the turbulence level of the waves, where they are tensioned by means of a subsurface buoy. Their upper end is connected to flexible risers allowing to carry the fluids to a floating support. These risers take up the differential motions between the support and the buoy. There are other versions of this configuration where the rigid risers are catenary risers such as those described in U. S. Pat. No. 5,639,187.